Low-pressure electric discharge device



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Patented June 21, 1949 UNITED STATES PATENT OFFICE LOW-PRESSURE ELECTRIC DISCHARGE DEVICE Clifton G. Found and Wilford J. Winninghoff, Cleveland Heights, Ohio, assignors to General Electric Company, a corporation of New York Application January 9, 1948, Serial No. 1,454 8 Claims. (Cl. 176-122) Our invention relates to electric discharge devices of the low pressure positive column type, such as fluorescent lamps and low pressure positive column discharge devices for producing ultraviolet radiation, and methods of operating such lamps and devices.

It is an object of our invention to provide new and improved low pressure positive column electric discharge devices and methods of operating such devices.

It is another object of our invention to provide new and improved low pressure positive column discharge devices, such as fluorescent lamps and generators of ultraviolet radiation, which afford greater efilciencies in operation than that afforded by the prior art devices.

It is a further object of our invention to provide new and improved electric discharge devices which permit the production of the same or greater amounts of radiation by using less power, or which produce the same amount of radiation in smaller structures.

It is a still further object of our invention to provide new and improved fluorescent lamps which are of smaller physical size for the same electrical specifications and radiation output, and which make possible great savings in manufacturin cost thereby resulting in a lower selling price of such lamps incident to the economy resulting from smaller envelope si..e and smaller amounts of fluorescent material needed.

In the fluorescent lamps and ultraviolet generating lamps, such as germicidal lamps, commercially available at the time of filing this application, argon is generally used as a filling gas with mercury in order to facilitate starting of the devices and to permit the devices to operate with a cathode voltage drop which is below the disintegration voltage. For example, in many of these prior art devices the nominal argon filling gas pressure is about 3.5 mms. If the pressure is reduced much below this value, the life of such de-. vices is shortened because of an increase in cathode voltage drop during starting at such low gas pressures. On the other hand, if the argon pressure is increased above the 3.5 mm. value, there is a decrease in efliciency, such decrease in efiiciency for a device of a given physical size being due to an increased voltage drop in the device which accompanies the increase in pressure.

For convenience in describing our invention and its advantages over the prior art lamps reference is made to the now conventional 40 watt fluorescent lamp having thermionic electrodes and which has an operating voltage of about 108 volts and an operating current of about 0.4 ampere. It is to be understood that this reference to a particular lamp does not limit the scope of application of our invention, but is employed merely as a convenient comparison basis. The manner in which the power input to a low pressure positive column type electric discharge device is consumed may be considered by referring to a low pressure positive column fluorescent lamp such as the prior art 40 watt fluorescent lamp, having an envelope length of 43 inches and a diameter of 1% inches, and in which the ionizable medium comprises mercury and argon at a pressure of 3.5 mms. of mercury. Analysis shows that the 40 watts input is consumed as follows:

(1) The energy loss at the electrodes which, in the lamp under consideration, is about 6 watts.

(2) Losses due to ion and electron recombination on the wall of the enclosing envelope and which is a continuously replenishing process. This is the energy consumption which is essential to permit the current to pass through the discharge column, and'in the 40 watt lamp amounts to slightly less than about 1 watt. The energy of the electrons in the discharge adjusts itself to a value which is just sufficient to compensate for the ion and electron losses to the envelope wall.

(3) Approximately 20 per cent of the 40 watts supplied, that is 8 watts, is lost in the positive column of the ionizable medium comprising mercury and the argon. This loss may generally be referred to as the gas losses.

When equilibrium of wall losses and ion generation is reached, the energy of the greater proportion of the electrons is below the minimum excitation and ionizing potential of the filling gas. These electrons however are able to excite the mercury vapor producing the following radiation:

(4) 22 watts of 2537 Angstrom unit radiation, which excites the fluorescent material to produce the visible radiation.

(5) 2 watts of 1847 Angstrom unit radiation.

(6) about 1 watt of visible radiation directly.

I In accordance with our invention we have found that radical improvements may be made in low pressure positive column electric discharge devices by using as the ionizable medium, which supports the arc discharge, mercury and an inert gas of the group consisting of krypton and xenon, or mixtures thereof, so that the quantity of radiation is a function of the lamp current, and .so that the maximum or optimum quantity of radiation is obtained by correlating the wall loading of the enclosing envelope to the inert starting ae'raoca l 3 gas pressure. In this manner, the operating temperature of the device and the ionizaoie medium pressure are controlled to obtain operation of the device within a region of the radiation output-envelope temperature characteristic in which the radiation output does not vary more than per cent with respect to themaximum value of the latter characteristic.

More specifically, we have found that in low pressure positive column electric discharge devices having given electrical specifications with respect to power input, that is, volta e and current supplied to the device terminals, we may obtain in a device having the same or given envelope diameter and which is 50 per cent longer, substantially the same amount of radiation per unit length, and therefor obtain a material increase in eiliciency. Alternatively, in employing the teachings of our invention we may construct a low pressure electric discharge device, having specified voltage and current consumption, of the same length but with a smaller diameter than that of a reference device and obtain the same amount of radiation therefrom, thereby making it possible to obtain a device of the same or greater radiation output at lower cost due to the reduction in bulb size or diameter. Thirdly, in accordance with our invention we may construct low pressure positive column electric discharge devices to have specified voltage and current ratings and wherein the lamp afiords greater efilciency.

In all of the above described variations of the instant invention the results stated are obtained by correlating the pressure of krypton, xenon, or mixtures thereof, to the wall loading of the envelope area. For example, in accordance with the teachings of our invention if the pressure of the inert gas filling does not exceed 12 mms. of mercury. and if the wall loading of the envelope surface lies within the range from}? to 21 milliwatts per square centimeter, inclusive, the above stated improvements are obtained.

For a better understanding of our invention reference may be had to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims. Fig. 1 of the accompanying drawing illustrates an embodiment of our invention as applied to a low pressure positive column fluorescent lamp. Fig. 2 is a set of curves showing the radiation output-lamp life characteristic for a fluorescent lamp constructed in accordance with our invention for difierent pressures of krypton employed as the fllling gas. Fig. 3 is a radiation output-bulb or envelope temperature characteristic for such a lamp; and Fig. 4 is a radiation output-current characteristic showing the relationship between radiation or lumens output with respect to current for diiferent constant values of mercury vapor pressure.

Fig. 1 illustrates a low pressure positive column fluorescent lamp, which isone type of device to which our invention may be applied. The lamp comprises an enclosing envelope I, constructed of glass, quartz or other material pervlous to the radiation to be emitted, and having positioned therein spaced electrodes 2 and 3, which may be of the thermionic or filamentary type, although our invention is not limited to this type of electrode, and may be used with electrode constructions of any type or configuration whether operated as hot or cold electrodes, and whether activated or not. The electrodes 2 and 3 illustrated may be of the filamentary type constructed of a refractory metal, such as tungsten, and which may be provided with activating coatings of an alkaline earth metal, such as oxides or carbonates thereof. The electrodes 2 and 3 may be supported by lead-in wires 4, I and 8, 1, respectively, which also serve as electrical connections to the electrodes from externally accessible contact pins II, II and l2, I: which are supported by bases I and 9. The use of two pins at each end of the lamp for connection to the electrodes is, of course optional, the form of such structure depending upon the nature of the electrodes employed.

Within the envelope I we employ as an ionizable medium a quantity of mercury, indicated by the globule I4 and a filling or starting gas, which is an inert gas from the group consisting of krypton and xenon, or mixtures thereof. The quantity of mercury used may be somewhat in excess of that required duringnormal operation of the lamp; and the pressure of the mercury vapor during operation may range from about 3 to 20 microns, having a cold pressure of about 1 to 3 microns. .The stated operating range of mercury vapor pressure corresponds approximately to the preferred envelope temperature operating range of 30 C. to 50 C. indicated in Fig. 3.

By correlating the pressure of the krypton or xenon, or mixtures of these gases, to be not greater than about 12 mms. of mercury, with respect to the wall loading to lie within the range from 7 to 21 milliwatts per square centimeter, inclusive, we obtain improvements and increases in the quantity of radiation, and also obtain very substantial improvements in the efiiciency of production of such radiation, whether it be ultraviolet radiation or visible radiation. Any suitable voltage or current controlling means, such as a variable-voltage leakage reactance auto-transformer may be com nected between an alternating current supply circult and the lamp terminals to control the current supplied to the lamp.

In the operation of a fluorescent lamp, it will be appreciated that the mercury supporting the arc discharge serves primarily as the source of 2537 Angstrom unit line radiation which in turn excites a phosphor or fluorescent material II which is preferably placed on the interior surface of the envelope and which converts the invisible ultra-violet radiation (2537 Angstrom unit radiation) into visible radiation.

There have been at least tempts to use krypton as a filling gas in the place of argon in low pressure discharge lamps, ultraviolet generators and germicidal lamps, but these have been unsuccessful or impractical. These prior attempts have not been productive because in each case an attempt was made to produce a device of a given physical size and as of the same wattage as with argon. The radiation output of a low pressure discharge device, or the light output of a low pressure fluorescent lamp. does not increase linearly with current but tends toward a saturation value as the current is increased. If one attempts to equal the wattage of a lamp using argon as the starting gas by merely increasing the current. the gain in light (lumens) is insignificant due to the non-linear relation between light and current.

To compare lamps or discharge devices it is possible to use any one of four factors as the basis, which factors may be defined as: equal wattage. equal radiation or lumens output, equal currents and equal dimensions. Irrespective of which basis is used, devices constructed in accordance with three previous atour invention afford the above-described advantages.

The term wall loading as used herein means the energy or power dissipated per unit area, such as milliwatts per square centimeter, of the envelope area, not including the losses at the electrodes. The energy input to the positive column of a low pressure positive column electric discharge device is considered as the energy or power input to the lamp terminals minus the losses at the electrodes. The wall loading is the difierence between the above defined energy input to the positive column minus the energy radiated out of or from the lamp, all divided by the surface area of that portion of the lamp surrounding the positive column which in most instances is sub-- stantially the entire tubular envelope area. For example, in the case of a fluorescent lamp of the low pressure positive column type the energy radiated is the wattage value of the visible radiation or light emitted by the lamp. In the case of an ultraviolet generating lamp, such as a germicidal lamp, which produces 2537 Angstrom unit line radiation, the energy radiated from such a lamp is of course the number of watts of 2537 Angstrom unit radiation. In this manner, the

term wall loading takes into consideration all conversion losses, which latter term does not include electrode losses.

By incorporating the teachings of our invention in electric discharge devices, such as fluorescent lamps, we accomplish increases in lumen output, increased efiiciency, or lower lamp cost and higher efilciency for the same lumen output. In order to show the manner in which these advantages are obtained by constructing lamps in accordance with our invention, we will consider three cases:

(1) The construction of a lamp having specified current and voltage consumption.

(2) Construction of a lamp having agiven length and specified voltage and current consumption.

(3) Construction of a lamp having a specified definite size, i. e., length and diameter.

Where it is desired to construct a lamp having a specified current, the lumens output for this current may be determined from characteristics such as that generally shown in Fig. 4 where constant mercury pressure curves show the rela-- tionship between the relative lumens output and the current. For operation within the region to the left of the maximum value of the curve shown in Fig. 3, curves A and B of Fig. 4 represent mercury vapor pressures of increasing magnitude in the order named. For operation to the right of the maximum value in Fig. 3, the mercury vapor pressure relation is reversed, that is, curve A of Fig. 4 represents a higher value constant mercury vapor pressure than curve B. By using a wall loading of the envelope, incident to the losses in the positive column as above defined, to have a value lying within the range from 7 to 21 milliwatts per square cm., inclusive, and by using a gas filling of krypton or xenon, or mixtures thereof, having a pressure not greater than 12 mms., we eilect operation of the lamp within the region of the maximum value of the characteristic of Fig. 3, or within a region thereof between 30 C. and 50 C. in which the lumen output does not vary more than per cent of the maximum value for an ambient temperature of about 25 C. When constructed in this manner to have these features, by increasing the envelope length to a. value to obtain a required voltage, we find that there is provided a lamp having a length 50 per cent greater than the prior art lamps. and which has the same voltage and current input or consumption, and which provides about the same number of lumens per unit length or foot, and which furthermore provides increases in luminous efilciency ranging to about 15 per cent or more.

In those instances where it is desired to employ our invention in a lamp having a fixed length and given voltage and current ratings the same features as to gas filling pressure and wall loading may be incorporated in the lamp, the envelope area or diameter being determined to bring the voltage to the given value and the wall loading within the stated range from 7 to 21 milliwatts per square cm., inclusive. When krypton or xenon or mixtures thereof at the stated pressures are used the envelope diameter is decreased as compared with the prior art lamps, however, the

amount of light or the total lumens is at least equal to that of the prior art lamps, and offering the further advantage of at least equal or greater efficiency and reduced lamp cost due to substantial reductions in lamp envelope area and diameter and the amount of fluorescent material needed. 7

Lastly, by incorporating the feature of envelope wall loading and gas pressures of krypton or xenon, or mixtures thereof, we make it possible to construct lamps having a given voltage and current rating, and in which there is obtained the above stated improvements in efliciency and lamp economy.

When krypton is used as the gas filling, and where the krypton gas filling pressure is preferably within the range from 1 to 6 mms. of mercury, inclusive, the lamps so constructed show good maintenance, as evidenced by the curves shown in Fig. 2, for the different indicated values of pressure. These curves are not to be construed as indicating a limitation on the pressure range intended andbeneflcial, but to indicate merely the direction of variation due to changes in pressure.

The above described advantages incident to the use of krypton or -xenon do not sacrifice lamp performance. Even though it is possible to use lower pressures of krypton or xenon, compared with argon pressures, equal cathode protection is obtained due to the higher atomic weights of krypton and xenon. Because it is possible to use krypton pressures as low as 2 mms., and lower, as contrasted with 3.5 mms. of argon, the use of smaller quantities of krypton assists in offsetting the present higher costs of krypton as compared with argon,

We have observed from lamps constructed as described that there are no bands (end discoloration) in any lamps using krypton. Although the starting voltage of krypton lamps is somewhat higher than that for argon lamps, this feature is not a substantial disadvantage particularly where a starting device is used. 0n the other hand, the lower.operating voltage of krypton lamps gives them a distinct advantage in those instances where the open circuit voltage of the associated ballasts is important.

The inclusion of the above described principles in low pressure positive column fluorescent lamps uniformly effects a reduction in the amount of gas losses of the positive column, and of course results in improved eillciency and in substantial increases in the amount of visible radiation obtainable. The increase in the amount of visible radiation obtained is, of course. due to the increased emciency in the production of the 2537 Angstrom unit line radiation which excites the Phosphor. For instance, in one type of argonmercury lamp investigated where the gas losses in the positive column were 8 watts, by the use of krypton at a pressure of about 2 ms, the gas losses have been reduced to 2 watts. As another example, in a 100 watt argon-mercury fluorescent lamp, by incorporating the above described principles, the same amount of light, that is visible radiation, is obtained requiring only 85 watts input to the lamp, resulting in a very substantial improvement in efliciency.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In an electric discharge device the combination comprising an envelope, electrode means within said envelope and an ionizable medium therein consisting of mercury and an inert gas of the group consisting of krypton and xenon,

./or mixtures thereof, said inert gas pressure not exceeding 12 millimeters of mercury, and the 'wall loading of the envelope surface lying within the range from 7 to 21 milliwatts per square centimeter, inclusive.

.2. In an electric discharge device the combination comprising an envelope, electrode means within said envelope and an ionizable medium therein consisting of mercury and an inert gas of the group consisting of krypton and xenon,

vary more than'5 per cent with respect to the maximum value of said characteristic.

5. In an electric discharge device of the low pressure positive column type for producing ultraviolet radiation the combination comprising an envelope, electrode means within said envelope and an ionizable medium therein consisting of mercury and an inert gas of the group consisting of krypton and xenon, or mixtures thereof, said inert gas pressure lying within the range from 1 to 12 millimeters of mercury and the wall loading of the envelope surface lying within the range from 7 to 21 milliwatts per square centimeter, inclusive.

6. The method of operating a low pressure positive column electric discharge device having spaced electrodes in an envelope and employing an ionizable medium consisting of mercury and a or mixtures thereof, said inert gas pressure not 7 exceeding 12-millhneters of mercury, and the wall loading of the envelope surface lying within the range from 7 to 21 milliwatts per square centimeter, inclusive, to establish a region of operation with respect to the radiation output-envelope temperature characteristic at which the output does not vary more than 5 per cent with respect to the maximum value of said characteristic.

3. In a low pressure positive column electric discharge device the combination comprising an envelope, electrode means within said envelope and an ionizable medium therein consisting of mercury and krypton, the pressure of said krypton lying within the range from 1 to 6 millimeters of mercury and the wall loading of the envelope surface lying within the range from 7 to 21 milliwatts per square centimeter, inclusive, to establish a region of operation with respect to the radiation output-envelope temperature characteristic in which the output 'does not vary more than 5 per cent with respect to the maximum value for ambient temperatures of about 25 C.

4. In a low pressure positive column electric discharge lamp of the fluorescent type the combination comprising a tubular envelope, electrode means within said envelope and an ionizable medium therein consisting of mercury and an inert gas of the group consisting of krypton and xenon, or mixtures thereof, said inert gas pressure lying within the range from 1 to 12 millimeters of mercury and the wall loading of the envelope surface lying within the range from 7 to 21 milliwatts per square centimeter, inclusive, so that said discharge device operates within a region of the radiation output-envelope temperature characteristic in which the lumen output does not gas from th group consisting of krypton and xenon at a pressure not exceeding 12 millimeters, which comprises controlling the current to obtain a wall loading of the envelope to have a value within the range of 7 to 21 milliwatts per square centimeter.

7. The method of operating a low pressure positive column electric discharge device having spaced electrodes in an envelope and employing an ionizable medium consisting of mercury and a' gas from the group consisting of krypton and xenon at a pressure not exceeding 12 millimeters, which comprises controlling the current to obtain a wall loading of the envelope to have a value within the range of 7 to 21 milliwatts per square centimeter and to effect operation within the region of the radiation output-envelope temperature characteristic near the maximum value thereof.

8. The method of operating an electric discharge device oi. the type employing spaced electrodes in an envelope and employing an ionizable medium consisting of mercury and a starting gas from the group consisting of krypton and xenon at a pressure not exceeding 12 millimeters, which comprises controlling the power input to said device to obtain a wall loading of the envelope lying within the range from 7 to 21 milliwatts per square centimeter, inclusive, and for eflecting operation of the device within the region of the radiation output-envelope temperature characteristic in which the output does not vary more than 5 P r cent with respect to the maximum value of said characteristic.

CLIFTON G. FOUND. WILFORD J. WINNINGHOFF.

REFERENCES CITED The following referenlces are of record in the file of this patent:

UNITED STATES PATENTS 

